1. Field of the Invention
The present invention relates to a hologram recording method and device, and in particular, to a hologram recording method and device which record a reflection-type hologram by illuminating signal light and reference light on the same axis.
2. Description of the Related Art
A method of fabricating a transmission-type hologram by illuminating signal light and reference light on the same axis from the same surface side of a recording medium has been proposed (see Japanese Patent No. 3452113). In this method, reference light and signal light generated from spatially different positions of a spatial light modulator are Fourier transformed by a lens. Because the Fourier transformed signal light and reference light are superposed in a vicinity of the Fourier transform plane, by placing the recording medium at this position, a hologram can be recorded. Further, in this method, because the signal light and the reference light are illuminated on the same axis, the optical system is simple, and the recording device can be made to be compact.
However, the further away, in the direction of the optical axis, from the Fourier transform plane, the smaller the region at which the signal light and the reference light are superposed. Accordingly, in same-axis recording of a transmission-type hologram, there is the major problem that, if the film thickness of the recording material is large, a hologram cannot be recorded in the entire optical axis direction (direction of thickness) of the recording medium.
In a volume hologram in which interference fringes are recorded three-dimensionally by utilizing the direction of thickness of the recording medium, the greater the thickness of the material, the stricter the Bragg condition, and the larger the dynamic range can be made. Further, the stricter the Bragg condition and the larger the dynamic range, the greater the number of holograms that can be multiple-recorded. Accordingly, the greater the thickness of the material, the greater the number of holograms that can be multiple-recorded.
Moreover, in order to make the recording density large, the recording region must be made to be small. In order to make the recording region small, it is desirable to make the focal length of the Fourier transform lens short. However, it is difficult to both make the thickness of the recording medium thick and make the focal length of the Fourier transform lens short. The reason for this is as follows: the recording medium exists at a position which is separated, in the optical axis direction, from the Fourier transform plane, and the shorter the focal length of the Fourier transform lens, the smaller the region at which the signal light and the reference light are superposed, even at a position which is slightly away from the Fourier transform plane in the optical axis direction. Therefore, the recording of a hologram is difficult at this position.
In order to realize a high recording density in this way, a thick recording material and a Fourier transform lens having a short focal length are needed. However, in same-axis recording of a transmission-type hologram, it is difficult to both make the thickness of the recording medium thick and make the focal length of the Fourier transform lens short, and therefore, it is difficult to realize high density recording.
Further, the aforementioned prior art proposes providing a reflective layer at the reverse surface of the recording medium. However, in this case, the quality of the signal light and the reference light is affected greatly by the quality of the reflective layer, and the problem arises that the quality of the signal light and the reference light deteriorates due to defects of the reflective layer or the like. Moreover, there is the problem that, in the reconstructing of data as well, it is easy for the SN ratio of the reconstruction light to deteriorate. The reason for this is that, because the reading light and the reconstruction light exit in the same direction on the same axis, scattering light of the reading light is incident on a detector together with the reconstruction light.
As a method of reducing this scattering light, Optics Letters Vol. 30 p 878-880 (2005) proposes sandwiching a recording material by quarter-wave plates. In this method, a reflection-type hologram is recorded by incident light of signal light and reflected light from the reflective layer of the reference light, and reflected light from the reflective layer of the signal light and incident light of the reference light. At the time of reconstructing, because the reconstruction light and the reading light are linearly polarized lights whose planes of polarization are orthogonal, the scattering light can be reduced by using an analyzer.
However, in this method, because quarter-wave plates are used at the recording medium, there are the problems that the recording medium is expensive and the production thereof also is complex. Moreover, when this recording medium is rotated around the optical axis, the optical axes of the quarter-wave plates become offset, and therefore, there is the problem that the recording medium cannot be used as a rotating-type medium such as a DVD.